Roading Slip Update

Roading Slip Repair Resolution

Council has assessed the priority of each slip on the network. While each slip carries a risk to the general public, we have strategically derived a list considering various factors and types of risk associated with each slip to produce a prioritised programme. Council will use this programme to get slips repaired and remedied accordingly. Those slips which are not of higher priority will be monitored actively and any change in condition will trigger a reassessment and reprioritisation if required. The programme will be updated on a quarterly basis.

Each slip remediation goes through a stringent process of investigation and construction. A brief scope on the process involved from initiation to completion is detailed below:

Geotechnical investigation on the cause of the slip

Engineering study on the possible remedial options

Seek funding for the recommended option to repair

Final design and tender documents

Tendering process

Close of tender and tender evaluation

Award of tender

Construction of repair/remediation

Construction review and issue of practical completion certificate

Defects liability period

Final completion certificate issued

Source

The Northland region consists mainly of low-lying, gently dissected terrain underlain by cretaceous and tertiary sediments. The Kaipara district has a mixture of geological ground conditions and is not stable when subjected to environmental, climatic and artificial (such as heavy loading) impacts. The ground is structured around mudstone and lime as shown in Figure 1.0, which are susceptible to movement when saturated.

On average the district receives 1000-1500 mm of rainfall annually. The terrain and the geology of the soil presents unpredictable conditions especially for the roading network. In April 2017 the district was hit by cyclones Debbie and Cook with 400mm of rainfall resulting in 23 slips. At the end of August 2017 the accumulated number of slips on the network total 211.

Timeline

Dependent on the solution and the function for each site, the timeframe can range from a few hours to do a temporary one-lane access, to a few months to design and construct a solution. With some slips it might be that they don’t get a permanent remedy at all.

Value

Council takes seriously providing the best and safest infrastructure for the general public’s use. The number of slips on the Kaipara network is high but each slips is under consideration and will be reviewed and remedied with time. Lane closures and restrictions are in place to keep motorists and the general public safe from hazards created by the slips. Council asks for your patience while we work together with the community to make our roads safer for all users.

Volcaniclastic sandstone, mudstone and conglomerate with local intercalated volcanic grit and breccia.

Melange of mixed Early Cretaceous to Early Miocene lithologies in a sheared mudstone matrix, locally with included tectonic blocks of local and exotic origin; associated with Early Miocene emplacement of the Northland allochthon.

Volcaniclastic sandstone and argillite with tectonically included basalt, chert and siliceous argillite.